In recent years fluidized beds have found many diverse uses in power generation systems and chemical processes. They have served as chemical reactors, particularly for finely divided materials; as incinerators for liquid, solid or gaseous substances; as pressurized or atmospheric, coal-, lignite-, petroleum-, peat-, wood- and/or paper-fired boiler or combustor units for power generation; and as sites for various process treatments such as drying, baking, coating, oxidizing, etc.
Typically, fluidized beds which are in use as combustor units are static beds established when air and a combustible fuel are introduced into a plenum chamber under pressure and forced upwardly through a diffusing medium (e.g., membrane, grate) to a superimposed chamber containing a particulate bed, of inert or reactive, finely divided, pulverulent solid material. The air, forced upwardly through the diffusing medium into the fluidizing chamber under a sufficient predetermined pressure, fluidizes the particulates. When the temperature within the bed is high enough to ignite the fuel-air reaction, combustion proceeds exothermically and, thus, the bed becomes self supporting.
In order to reach the combustion reaction ignition temperatures, ordinary fluidized bed combustors must be preheated. This is particularly true of combustors employing solid fuels, such as powdered coal. Generally, in order to initiate combustion, the bed is first fluidized by passing air through it from the bottom (upstream side of the bed). A preheating flame is applied to the upper bed surface (downstream side of the bed) which, due to the considerable mixing action which occurs in any fluidized bed, heats up the entire bed. As the ignition temperature is reached the mixture of fuel and air begins to support an exothermic combustion reaction which is self sustaining, and the preheating flame is withdrawn. Although this combustor start-up technique is effective, it is extremely wasteful. This is because the fluidizing air passed through the bed during preheating is an effective coolant for the bed. Thus, the passage of air works at cross purposes with the preheating procedure. To be sure the cooling effect of the air can be avoided by preheating it prior to passing it into the bed. However, this requires additional equipment and additional energy and is inefficient for these reasons.
The present invention is directed to overcoming one or more of the problems as set forth above.